To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case. This will especially lower the noise for the version with higher resistor to ground (e.g. 2 K / 20 K and 100 µF cap). Usually signal sources are kind of low impedance compared to the resistor at the input - if not, one has to include the reduced gain / source loading. So the open circuit test is misleading.
The gain is expected to be linear down to low amplitudes. There are quite a few resistors that reduce the gain a little, but the simulation should tell.
Both the 1K version and 2/20K versions had -3dB points of ~0.1-4Hz.
Settling time each time a measurement is started is long…often taking a minute or so for both op-amps to come out of saturation and settle near the baseline.
To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case.
Both the 1K version and 2/20K versions had -3dB points of ~0.1-4Hz.
Settling time each time a measurement is started is long…often taking a minute or so for both op-amps to come out of saturation and settle near the baseline.
probably the additional low pass R8/C16? could be the reason.
R8 (10K) + the following 1K (R20?) resistor are larger than the 3K6 in my cirquit.
It is clear (to me) that the settling time is long for a 0.1 Hz lower frequency corner.
(Time constant 3.3 sec and at least 10-20 Tau to settle to uV level)
Andreas
The relevant case is with a low noise around 10V source (e.g. 8*NiMH AA cells at constant temperature).
Otherwise you do not get the real life noise due to leakage current of the input capacitor.
A short or open will give too low values for the noise floor.
with best regards
Andreas
Electrolytic caps have a rather long internal settling time due to dielectric absorption. So independent on the capacitance value it is just the capacitor internal that takes a long time to settle. Depending on the capacitor type this can take rather long - well in to the seconds or even minutes for the time constant. Checking the settling / dielectric absorption could be another parameter in addition to leakage to look at when choosing caps.
If it would be just the RC time-constant at the input and a later filter stage, there would be the option to speed things up, but reducing the resistor for the initial settling phase.
The LT1037 should be very fast to come out of saturation (more like < 1 µs range). An AZ OP like the ADA4522 usually takes a little longer (10 µs - a few ms) - especially the modern ones with higher GBW may be a little faster. The difference is more like the difference film cap versus electrolytic cap.
The real world test is with a low noise voltage source - especially with an electrolytic capacitor. With a film capacitor the extra noise compared to a short should not be much, but with an electrolytic the extra noise due to leakage can be important. The open input could give a higher noise, as the resistor to ground in not shunted paralleled with the signal source. So it depends on the input capacitor if the open input noise is higher of lower than the real test with a voltage source.
To measure the noise of the amplifier itself, the short circuit case is more relevant than the open case.The relevant case is with a low noise around 10V source (e.g. 8*NiMH AA cells at constant temperature).
Otherwise you do not get the real life noise due to leakage current of the input capacitor.
A short or open will give too low values for the noise floor.
Andreas
I bought 8x NiMH AA (EverReady) cells from Digikey and charged them with a new T4S Tenergy charger bought for this purpose. I found the output voltage surprising noisy as shown in an attached scope waveform of the output on my 10K:1 amp.
Newly charged batteries might show a little more noise and drift. It can take some time for the chemistry to settle and this might not always be smooth. Similar it might take quite some time for the input cap to settle with the new voltage.
It is very unusual to need filtering after the battery.
It is very unusual to need filtering after the battery.
It is very unusual to need filtering after the battery.
Thats true.
I also think that the input capacitor was not charged long enough. (2-3 days)
I always keep the input capacitor charged to about 10V except for noise measurements.
With best regards
Andreas